1. Field of the Invention
The present invention relates to a taking lens apparatus for a camera, and more particularly to a vari-focal lens apparatus in which one of plural lenses is selectively set at an optical path and a focal length is changed stepwise.
2. Description of the Related Art
As to cameras in which a photographic angle of view is changeable, there are a two-focus setting camera and a zoom camera. With respect to the former, there are two systems. In one of which, any one of plural taking lenses is selected to be set at an optical path, and in the other of which, a tele conversion lens is removably disposed behind a normal lens. Meanwhile, the latter employs a zoom lens apparatus in which a focal length is successively changed. In a motor drive type, it is general to select plural focal lengths within a range from several tens to several hundreds.
Although the zoom lens apparatus successively changes the focal length, the focal lengths actually used are almost both sides (a wide-angle side and a telephoto side) and one or two points of a central portion. Recently, a simplified zoom camera is put in the market. Such camera has a zoom lens apparatus which is proper to be regarded as a vari-focal lens apparatus and in which its focal length is changed in four or five steps. The above zoom lens apparatus is called as a step-zoom-lens apparatus, and upon rotating a lens barrel, changing the focal length and focusing are alternately performed. In the step-zoom-lens apparatus, it is sufficient to architect a cam structure for moving a lens such that objective performance is achieved relative to only four or five focal lengths. Accordingly, a lens barrel and a focusing mechanism are simplified so that it is possible to decrease the cost of the camera.
However, the step-zoom-lens apparatus is the zoom lens apparatus in effect. Thus, the step-zoom-lens apparatus has a plurality of cam barrels in which a complicated cam groove for zooming is formed. For this reason, complexity of the structure can not be avoided in some extent so that there is a limitation for decreasing the cost. In order to vastly decrease the cost with secureness of necessary performance, it is required to develop a new lens apparatus which is entirely different from the conventional structure.
It is a primary object of the present invention to provide a vari-focal lens apparatus which has performance being identical with that of a step-zoom-lens apparatus, and in which its structure is simplified, a space is reduced, and a cost is vastly decreased.
It is another object of the present invention to provide a vari-focal lens apparatus in which focusing can be performed in a state that a selected focal length is kept.
It is the other object of the present invention to provide a vari-focal lens apparatus in which changing a focal length and focusing are alternately performed by moving a lens barrel.
It is the other object of the present invention to provide a vari-focal lens apparatus in which a camera body may be downsized by attaching a focal-length changing motor to a lens barrel.
In order to achieve the above and other objects, the present invention comprises a lens turret to which plural lens groups having different focal lengths are attached. The lens turret is rotatably disposed in a lens barrel moving by a predetermined stroke. This stroke of the lens barrel is divided into focusing sections and focal-length changing sections positioned between them. Each of the focal-length changing sections is set, taking account of back focus of the selected lens group. When the lens barrel passes through the focal-length changing section, the lens turret is intermittently rotated. Upon rotation of the lens turret, one of the lens groups is set at an optical path to change the focal length. In the focusing sections, the lens turret is advanced without rotating by means of a positioning member, and focusing is performed stepwise under the selected focal length.
The respective lens groups of the lens turret are arranged in accordance with the focal length so as to be located at the optical path on account of the back focus. It is preferable that the respective lens groups adopt a single construction for decreasing its cost. In order to improve aberration correction and lens performance, a lens group for correcting aberration is disposed in front of and/or behind the lens turret. The lens for correcting the aberration is commonly used relative to all lens groups of the lens turret to reduce a number of parts. The lens turret and the lens groups are integrally formed from a transparent plastic in order to decrease the cost. Incidentally, it is desirable that the lens group for correcting the aberration adopts a single construction.
A rotational center of the lens turret is shifted from the center of the lens barrel. The intermittent rotation of the lens turret is performed by a protrusion and a feed claw which are intermittently engaged with each other. The protrusion is provided on a lens holder and the feed claw is provided on the lens barrel. When the lens turret is rotated, the desired lens group approaches the optical path. At this time, however, the engagement of the feed claw and the protrusion has been already released. Thus, under this state, the lens group does not stop at a proper position. Meanwhile, the lens turret has a cam portion and is pressed by a positioning member when freed from the lens barrel. The lens turret is further rotated to correctly set the desired lens group to the optical path. Incidentally, the positioning member prevents the lens turret from rotating in vain while focusing is performed.
The lens barrel comprises a first movable barrel and a second movable barrel. The first movable barrel moves in an optical axis direction in a rotating state. The second movable barrel goes straight in the optical axis direction together with the first movable barrel without rotating. The second movable barrel holds the lens turret in a rotatable manner and is rotatably attached to the first movable barrel. By rotating the first movable barrel, the lens turret goes straight in the optical axis direction together with the second movable barrel, and is rotated at a predetermined position. The first movable barrel is rotated by a single motor to move for both of changing the focal length and focusing. The motor is attached to the second movable barrel so that light shielding for the lens barrel is secured by a simple structure. Incidentally, it is possible to use a stepping motor, a DC motor with a rotary encoder, and so forth.
The first movable barrel moves, rotating in a fixed barrel. The fixed barrel holds a cylinder cam in a rotatable manner. The cylinder cam is formed with a first cam groove and a second cam groove. The first cam groove is a succession of a straight line or a curved line, and the second cam groove has a zigzag shape. The first cam groove engages with a projection provided on the second movable barrel, and the cylinder cam is rotated by the straight movement of the second movable barrel. The second cam groove engages with a cam pin of the lens holder for keeping a movable lens of a viewfinder lens system. When the cylinder cam is rotated in association with the change of the focal length, the movable lens of the viewfinder lens system is moved stepwise to change a viewfinder magnification in accordance with the focal length of the vari-focal lens system. While focusing in the vari-focal lens system is performed, the movable lens of the viewfinder lens system is stopped so that a prescribed magnification of the viewfinder is maintained.
According to the present invention, changing the focal length is performed by intermittent rotation of the lens turret caused by movement of the lens barrel. Moreover, focusing is performed during straight movement after the intermittent rotation. Thus, in comparison with a conventional step-zoom-lens apparatus, the structure is extremely simplified. Further, the whole of the lens turret is constructed as a single unit, and the lens for correcting the aberration is commonly used for the respective lens groups of the lens turret. Therefore, the cost may be vastly decreased. In this way, it becomes possible to provide the vari-focal lens apparatus which has performance being identical with that of the step zoom apparatus, and in which the structure is simplified, a space is reduced, and the cost is vastly decreased.
Moreover, by adopting a combination of a worm and a worm wheel as a reduction gear train, a large reduction gear ratio is obtained by a small number of gears. Thus, it is possible to extremely downsize the reduction gear train. Owing to this, the motor can be built in the lens barrel so that light shielding for the lens barrel may be secured by a simple structure. Further, the whole of the camera may be downsized. Furthermore, a number of parts may be reduced.